Electron emission device

ABSTRACT

The invention provides an electron beam device  1  comprising at least one field emission cathode  3  and at least one extracting electrode  5,  whereby the field emission cathode  5  comprises a p-type semiconductor region  7  connected to an emitter tip  9  made of a semiconductor material, an n-type semiconductor region  11  forming a pn-diode junction  13  with the p-type semiconductor region  7  a first electric contact  15  on the p-type semiconductor region  7  and a second electric contact  17  on the n-type semiconductor region  11.  The p-type semiconductor region  7  prevents the flux of free electrons to the emitter unless electrons are injected into the p-type semiconductor region  7  by the pn-diode junction  13.  This way, the field emission cathode  3  can generate an electron beam where the electron beam current is controlled by the forward biasing second voltage V 2  across the pn-diode junction. Such electron beam current has an improved current value stability. In addition the electron beam current does not have to be stabilized anymore by adjusting, the voltage between emitter tip  9  and extracting electrode  5  which would interfere with the electric field of electron beam optics. The present invention further provides the field emission cathode as described above and an array of field emission cathodes. The invention further provides a method to generate at least one electron beam.

FIELD OF THE INVENTION

[0001] The invention relates to field emission cathodes or arrays offield emission cathodes. It also relates to electron beam devices withfield emission cathodes or with arrays of field emission cathodes, andto methods of generating electron beams.

BACKGROUND OF THE INVENTION

[0002] Field emission cathodes and arrays of field emission cathodes areknown electron beam sources for electron beam devices in applications asdiverse as e.g. electron microscopy, electron pattern generators or flatpanel displays.

[0003] Field emission cathodes emit electrons into free space byapplying a high electric field to the surface of the emitter tip of thefield emission cathode. Without electric field there is usually apotential barrier of theoretically infinite thickness at the interfaceof the emitter tip and free space or vacuum.; The height of thepotential barrier depends on the surface material of the emitter tip.When an external electric field is applied to the emitter tip thatattracts electrons, the potential barrier thickness reduces. When theelectric field at the surface of the emitter tip is larger than ca. 10⁸V/m, the potential barrier thickness reduces to a level where electronsin the emitter tip succeed in tunneling through the potential barrierinto free space. This phenomenon is called field emission, in contrastto electron emission caused by e.g. thermal excitation, photo-effectetc.

[0004] Usually the high electric field is generated by applying avoltage between the emitter tip and an extracting electrode facing theemitter tip. In order to achieve sufficient field strength at theemitter tip, the electron emitting surface of the emitter is in theshape of a sharp tip (tip radius typically 1 nm to 100 nm). The emittertip is usually made of metal or semiconductor material.

[0005] Among the many advantages of field emission cathodes compared tomore traditional electron beam sources, like e.g. tungsten hairpinfilaments, are their small emission source size, which is important forelectron beams-used for precision focussing applications, their superiorbrightness, a smaller energy spread of the electrons within the electronbeam and a longer lifetime. However, field emission cathodes also havedrawbacks because of their need for high vacuum and because of a poorelectron emission current stability.

[0006] The electron emission current instability is understood to becaused by the extreme sensitivity of the electron emission current onchemical or physical changes of the surface of the emitter tip. With theemitter tip having an apex radius of typically only a few nanometers, adeposition of a few atom layers or tiniest deformations of the apexduring operation can cause significant electron emission current changesduring operation. Many applications like e.g. electron microscopy,e-beam pattern generators, and other precision devices, require a highelectron beam current stability.

[0007] To achieve a better electron emission current, some effort hasbeen made to actively regulate the electron emission current byadjusting the voltage between emitter tip and extracting electrodeaccording to the changes of the electron emission current. However thisconcept has the drawback that for electron beam precision devices likeelectron microscopes, the voltage changes between extracting electrodeand emitter tip interfere with the electric field of the electron beamoptics. Such interference can deteriorate the focussing capabilities ofprecision electron beam devices.

[0008] For some time large arrays of field emission cathodes have beenintegrated onto semiconductor substrates using semiconductormicroprocessing techniques. Semiconductor microprocessing techniquesallow large arrays of micron-size field emission cathodes to befabricated onto minimal surface area. In addition, extraction electrodesand/or electronic control circuits for each field emission cathode canbe integrated onto the semiconductor substrates in a cost-effective way.Arrays of field emission cathodes are seen to have large commercialpotential for many applications, e.g. for flat panel displays as well asfor electron microscopy or e-beam pattern generators where paralleloperating electron beams can dramatically improve the processingthroughput.

[0009] The fabrication of field emission cathodes on semiconductormaterial has several advantages. One reason is that the fabrication ofemitter tips from a semiconductor substrate, especially from siliconsubstrates, is straightforward. Furthermore, semiconductor emitter tipscan be doped in order to adjust their electronic properties to a givenapplication. In particular it has been found that the choice of thepolarity of the majority carrier of the respective semiconductormaterial has a profound impact on the emission behavior of emitter tips:n-type semiconductor emitters connected to some voltage source likemetallic emitters emit electrons according to the Fowler-Nordheimformula; in contrast p-type emitters connected to some voltage sourcedeviate from the Fowler-Nordheim formula significantly.

[0010] The different electron emission current behavior of p-typeemitters is thought to be caused by the absence of electron abundance inp-type emitters. Therefore, the emission current can be limited by thenumber of free electrons in the p-type material, and not by thepotential barrier at the surface of the emitter tip. This is contrary tothe model of Fowler-Nordheim, where the electron emission current islimited by the potential barrier at the emitter surface.

[0011] A detailed study of the different behaviors of p-type emittersand n-type emitters has been performed, e.g. in “Control of emissioncurrents from silicon field emitter arrays using a built-in MOSFET” bySeigo Kanemaru et.al., Applied Surface Science 111 (1997),p.218-223 or“The Semiconductor Field-Emission Photocathode” by Dieter K. Schroderet.al., IEEE Trans. Electr. Dev., Vol ED-21, No 12, Dec. 1974.

[0012] In “The Semiconductor Field-Emission Photocathode” by Dieter K.Schroder et.al., the electron emission current limiting effect has beenused to design a p-type field emission cathode where the emission rateis controlled by external light that generates electron-hole pairs inthe p-type emitter region via photo-effect. The generated electronsdiffuse until they recombine or arrive at the emitter surface where theycan be emitted with an external electric field. The strength of theexternal field is so high that, in this model, the emission current islimited by the number of free electrons generated by the external lightintensity and not by the tunneling probability through the potentialbarrier.

[0013] An important advantage of generating an electron beam currentthrough light excitation is that the electron beam current can becontrolled by the external light intensity without changing the voltagebetween extracting electrode and emitter tip. This avoids the mentionedproblem of interfering with the electric fields of the electron beamoptics used for high precision electron beam devices.

[0014] The p-type field emission cathode with light excitation howeverhas severe limitations. For one thing, it is costly to install a lightsource near a field emission cathode with a beam that points to thesmall emitter tip region. Even when light is coming from behind of thesubstrate as shown in the above-mentioned paper by D. Schroder, it isdifficult to control the stability of the light power to the extentneeded for a well-controlled electron beam current. Finally, for a largearray of field emission cathodes integrated onto a substrate there seemsno easy way to control the emission current individually by the use ofexternal light sources.

SUMMARY OF THE INVENTION

[0015] The present invention intends to provide improved electron beamdevices, improved field emission cathodes, improved arrays of fieldemission cathodes as well as improved methods for controlling electronbeams. According to one aspect of the present invention an electron beamdevice is provided as specified in independent claim 1.

[0016] According to a second aspect of the present invention an electronbeam device is provided as specified in independent claim 12.

[0017] According to a third aspect of the present invention a method forcontrolling at least one electron beam is provided as specified inindependent claim 18.

[0018] According to a fourth aspect of the present invention a fieldemission cathode is provided as specified in independent claim 31.

[0019] According to a fifth aspect of the present invention a fieldemission cathode is provided as specified in independent claim 32.

[0020] According to a sixth aspect of the present invention an array offield emission cathodes is provided as specified in independent claim39.

[0021] Further advantages, features, aspects and details of theinvention are evident from the dependent claims, the description and theaccompanying drawings. The claims are intended to be understood as afirst non-limiting approach of defining the invention in general terms.

[0022] The invention according to claim 1 and 12 provides an electronbeam device with at least one field emission cathode and at least oneextracting electrode, where the electron beam current can be controlledby the second voltage V2 across the pn-diode junction. By having ap-type semiconductor region connected to the emitter tip and byproviding a sufficiently high electric field at the surface of theemitter tip the electron beam device can be operated in a mode where theelectron emission current is limited by the current injected into theemitter tip through the pn-diode junction. This mode is calledsaturation mode.

[0023] In the saturation mode the emission current is predominatelycontrolled by the electrons injected into p-type semiconductor region,which preferably is controlled by the second voltage V2 across thepn-diode.

[0024] According to the invention, the p-type semiconductor region isconnected to the emitter tip whereby an electron current entering theemitter tip flows through the p-type semiconductor region. This impliesthat the current entering the emitter tip is determined by the currentthat the p-type semiconductor region delivers to the emitter tip. Sincep-type semiconductor material per se has essentially no free electrons(except for leakage current) the electron current that the p-typesemiconductor region can deliver to the emitter tip preferably dependson the electron current that the n-type semiconductor region injectsinto the p-type semiconductor region via the forward biased pn-diode.The electron current delivered to the emitter tip for electron emissionaccordingly depends on a forward biasing voltage across the pn-diodejunction.

[0025] The p-type semiconductor region has a first electric contact,while the n-type semiconductor region has a second electric contact.Both electric contacts serve to be able to apply a second voltage V2across the pn-diode junction for controlled current injection.Preferably, both electric contacts are ohmic contacts with a lowresistance in order to have good voltage control across the pn-junction.The first electric contact also serves to apply a first voltage V1between the emitter tip and the extracting electrode which defines theelectric field at the emitter tip during operation.

[0026] The electron current entering the emitter tip preferably flowsthrough a non-depleted p-type portion. Preferably, the non-depletedp-type portion of the p-type semiconductor region is in ohmic contactwith the first electric contact. This way the voltage of thenon-depleted p-type portion of the p-type semiconductor region can becontrolled by the voltage of the first electric contact. This allows theelectron current between the n-type semiconductor region and the p-typesemiconductor region to be controlled by the second voltage V2.

[0027] In the saturation mode the first voltage V1 is so high that theelectron emission current is limited by the pn-diode junction current.Preferably, the electron current through the pn-diode junction isdetermined by the second voltage V2 across the pn-diode. The saturationmode therefore offers many advantages over previously known fieldemission cathodes: firstly, emission current instabilities due tochanges of surface states and shape of the sharp emitter tip duringoperation are suppressed; this is because in the saturation mode theelectric field at the surface of the emitter tip is so high thatinjected electrons are emitted into free space independently, whetherthe surface state or shape of the emitter tip changes during operationor not. Instead, the electron beam current is determined by the electroncurrent injected into the p-type semiconductor region and preferably bythe second voltage V2 across the pn-diode, which can be controlled to avery high level of stability.

[0028] Secondly, the electron emission current can be controlled withoutchanging the voltage between extracting electrode and emitter, which isimportant for applications such as electron microscopy or electron beampattern generators. The focussing properties of high precision electronbeam optic systems deteriorate when voltage changes of the extractingelectrode or emitter tip interfere with the electrostatic fielddistribution of the electron beam optic system.

[0029] Thirdly, pn-diodes can be easily integrated into field emissioncathodes manufactured on a semiconductor substrate using microprocessingtechnology. Finally, the implementation of pn-diodes to arrays of fieldemission cathodes integrated on a semiconductor substrate isstraightforward.

[0030] The extracting electrode serves to generate a high externalelectric field at the emitter tip which is necessary to enable electronsto tunnel into tree space. Preferably, the extracting electrode facesthe emitter tip of the field emission cathode. Preferably, theextracting electrode faces the apex of the emitter tip to generate thehighest electric fields there. The apex therefore is preferably the onlyspot of a field emission cathode which emits the electrons. Its size canbe as small as a few nanometers in diameter. With increasing positivefirst voltage V1 at the extracting electrode with respect to the emittertip the electric field at the emitter surface increases, which decreasesthe thickness of the potential barrier. A decreasing thickness of thepotential barrier in turn increases the probability that electronstunnel from the emitter tip into free space.

[0031] Preferably, the extracting electrode is at an electric potentialwith respect to the emitter which is large enough to make electronstunnel through the potential barrier at a rate much faster than the rateat which electrons are injected into the emitter tip. The higher theemission probability given by the potential barrier between the surfaceof the emitter tip and free space the less the influence of changes ofsurface states or shape of the emitter tip during operation toinstabilities of the electron beam current. In other words, the higherthe emission probability given by the potential barrier between thesurface of the emitter tip and free space, the better the control of theelectron beam current through the voltage across the pn-diode. For thatreason, the extracting electrode is at a voltage with respect to theemitter which generates an electric field at the emitter tip preferablylarger than 10⁷ V/m and preferably larger than 10⁸ V/m.

[0032] The electron beam is made of the electrons emitted from theemitter tip into free space. While the electron emission current is thecurrent emitted from the emitter into free space, the electron beamrepresents the emitted electrons traveling along the direction of theelectric field. Usually the emitted electrons travel towards theextracting electrode unless other anodes with even higher potentials arewithin reach. For some electron beam devices the electron beam alsosplits in a way that some electrons travel towards the extractingelectrode while other electrons travel towards other anodes. In thiscase the electron beam current at the anode may be different from theelectron emission current at the emitter tip.

[0033] The p-type semiconductor region of the field emission cathodeserves several purposes. Firstly, it is in connection with the emittertip to deliver electrons for electron emission into free space. In thesaturation mode the electron emission current is equal to the electroncurrent delivered to the emitter tip by the p-type semiconductor, exceptfor leakage current in the emitter tip. Secondly, the p-typesemiconductor region serves as a material where electrons are minoritycarriers. Therefore, the p-type semiconductor region surrounding theemitter tip cuts off the emitter tip from electron sources other thanthose coming from the pn-diode junction. This feature enables thepn-diode in the saturation mode (and ignoring leakage current) to havefull control of the electron emission current. Thirdly, the p-typesemiconductor region represents the p-type portion of the pn-diode thatthe p-type semiconductor region forms with the n-type semiconductorregion. The pn-diode in turn is used, preferably as an electron source,to inject an electron current into the p-type emitter region. Fourthly,the p-type semiconductor region carries the first electric contact whicha) holds the emitter tip at a defined first voltage V1 with respect tothe extracting electrode; b) holds the non-depleted p-type portion at adefined second voltage V2 with respect to the n-type semiconductorregion; and fifthly, the p-type semiconductor region preferably is incontact with the non-depleted p-type portion through which the injectedelectrons have to diffuse to reach the emitter tip surface for emissioninto free space.

[0034] The emitter tip is the body connected with the p-typesemiconductor region which emits electrons when free electrons areavailable and when a sufficient first voltage is applied between thep-type semiconductor region and the facing extracting electrode.According to the invention, the emitter tip is made of a semiconductormaterial. Preferably, the semiconductor material is silicon. The emittertip may be doped with p-type or n-type material depending on the desiredelectron emission behavior and other desired features of the fieldemission cathode. For example, the polarity of the doping typedetermines the charge polarity of the majority carrier of the emittertip; if the emitter tip is p-type doped, the majority carriers arepositive holes and only few electrons are free for electron emission andvice versa. Further, the doping level of the emitter tip determines theresistance of the emitter tip. Further, when the apex of the emitter tipis p-type doped the doping level determines the size of the depletionregion. A low p-type doping level causes a large depletion region at theapex of the emitter tip when an external field is applied. A largedepletion region in turn may contribute to a large leakage current whichgenerates an emission current which cannot be controlled by the secondvoltage V2. Finally, the emitter tip preferably does not have anelectric connection to regions of the field emission cathode otherthan-the one to the p-type semiconductor region. This is to excludeelectron currents to the emitter tip which do not go through the p-typesemiconductor region.

[0035] Preferably the emitter is an outwardly pointing body on thesurface of the p-type semiconductor region. Preferably, the emitter tiphas a form similar to a circular cone or needle pointing into free spacewith a sharp apex. In,order to generate the highest possible electricfield strengths at a given voltage applied between p-type semiconductorregion and a facing extracting electrode, the ratio between the lengthof the emitter tip to the radius of the apex is preferably maximized.Preferably, the apex radius of the emitter tip has a radius smaller than200 nm and preferably smaller than 20 nm. Preferably, the ratio betweenemitter tip length and emitter apex radius is larger than 50 andpreferably larger than 500. The length of an emitter is typically givenby the distance between the apex to the base of the emitter tip, thelatter usually being in plane with the main surface of a substrate.

[0036] While the emitter tip 9 is made of a semiconductor material, thisdoes not exclude that there is a coating material on the emitter tipsurface which is made of material other than a semiconductor, e.g. ametal or an insulator. Preferably, the thickness of the coating materiallayer is smaller than 100 nm and preferably smaller than 20 nm in orderthat electrons can tunnel through the metal layer for electron emission.

[0037] In one preferred embodiment the coating material on the emittertip 9 is a layer of an insulator material Such insulator material e.g.may serve to passivate the emitter tip surface in order to reduce theleakage current. Again, preferably, the thickness of the insulator layeris smaller than 100 nm and preferably smaller than 20 nm in order thatelectrons can tunnel through the insulator layer for electron emission.

[0038] The first electric contact on the p-type semiconductor regionprovides an electric connection between the p-type semiconductor regionwith external voltage sources. The external voltage sources serve toprovide a first voltage V1 between the emitter tip with respect to theextracting electrode and a second voltage V2 between the p-type portionof the pn-diode with respect to the n-type portion of the pn-diode.

[0039] Preferably, the first electric contact is an ohmic contact. Anohmic contact is an electric contact whose resistance is independent ofthe current direction. Preferably, the resistance of the ohmic contactis so small that it does not significantly change the potential of theconnected region during the operation of the electron beam device.Typically, ohmic contacts on semiconductor material devices are realizedby a metal-semiconductor layer structure where the semiconductor in thecontact region is highly doped in order to reduce the resistance of thejunction between metal and semiconductor. An ohmic contact allows thep-type semiconductor region to be adjusted to a well defined voltage bysome external voltage source. In particular, with an ohmic connectionthe resistance between external voltage source and p-type semiconductorregion is largely independent of the current direction.

[0040] The n-type semiconductor region is adjacent to the p-typesemiconductor region to form the pn-diode junction with the p-typesemiconductor region. Furthermore, a second electric contact is on then-type semiconductor region, which preferably is an ohmic contact.Therefore, the electric potential of the n-type semiconductor region isdefined by a voltage applied to the second electric contact. A secondvoltage V2 between the first electric contact on the p-typesemiconductor region and the second electric contact on the n-typesemiconductor region defines the voltage across the pn-diode. The secondvoltage V2 therefore also determines the electron current that then-type semiconductor region injects into the p-type semiconductorregion. The injected electron current in turn determines the electroncurrent that the p-type semiconductor region can deliver to the emittertip for electron emission into free space. Therefore, in order toprovide a constant electron emission current, the second voltage V2across the pn-diode junction preferably is well controlled.

[0041] Preferably, the second electric contact is an ohmic contact, too,which is independent of the direction of the current and which keeps then-type semiconductor region at a well-defined potential during standardoperation. A stable voltage for the p-type semiconductor region as wellas for the n-type semiconductor region is extremely important in orderto bias the pn-diode precisely in order to control the current injectioninto the p-type semiconductor region with high precision. The tightcontrol of the current injection into the p-type semiconductor regionenables a well-defined electron beam current.

[0042] Preferably, the emitter tip is made of p-type material. This wayduring operation, the non-depleted p-type portion of the emitter tip isin ohmic connection with the p-type semiconductor region. This impliesthat the voltage of the non-depleted p-type portion of the emitter tipis determined by the voltage of the p-type semiconductor region insteadof by the external electric field generated by the extracting electrode.A constant voltage at the first electric contact therefore also providesa constant voltage at the non-depleted p-type portion of the emittertip, i.e. it does not depend on changes of the external electric fieldat the emitter tip due to chemical states or shape of the emitter tipduring operation. This is advantageous for electron beam devices whereelectric field interference into the electron beam region due tofluctuations of the emitter tip potential causes deterioration of theelectron beam device performance.

[0043] In order that the injected electrons reach the emitter tipsurface for electron emission they preferably have to travel through thenon-depleted p-type portion of the p-type semiconductor region andpossibly the non-depleted p-type portion of the emitter tip. The travelof electrons through the non-depleted p-type portion is critical sincein this region the recombination rate of electrons with holes is highdue to the abundantly available holes. Therefore, most electrons passthrough the non-depleted p-type portion at a region where the distancethrough the non-depleted p-type portion is at a minimum. The percentageof electrons that succeed in passing through the non-depleted p-typeportion therefore is characterized by the minimum non-depleted p-typedistance D. To achieve a high electron transport efficiency it isadvantageous to have the minimum non-depleted p-type distance D as shortas possible.

[0044] The desire for a high electron transport efficiency is comparableto the desire to design a bipolar npn-transistor with a high currenttransport factor. The transport factor of a bipolar npn-transistor witha base contact, emitter contact and collector contact is given by theratio of the collector currents to emitter current. There too, electronsare injected from the n-type emitter into a p-type base where theinjected electrons recombine with holes or diffuse toward the collector.In order to transport a large fraction of the injected electrons fromthe p-type base to the n-type collector the thickness of the base shouldbe much smaller than the diffusion length, L_(n), of electrons in thep-type base. Otherwise many or the majority of injected electronsrecombine in the p-type base with the abundantly available holes beforethey reach the collector.

[0045] The same holds true for the present invention where the minimumnon-depleted p-type distance D(i.e. the “base layer thickness”) ispreferably shorter than the diffusion length, L_(n), of electrons in thep-type semiconductor region. Preferably, the minimum non-depleted p-typedistance D is even 10 times shorter than the diffusion length, L_(n).This significantly reduces the loss of injected electrons due torecombination with holes, reduces electron emission noise fluctuationsand reduces the current through the first electric contact, whichincreases the stability of the operation of the field emission cathode.

[0046] A high electron transport efficiency requires a long diffusionlength, L_(n). Thereby, the diffusion length, L_(n), is known to be:

L _(n) =SQRT(kT×μ _(n)×τ_(n) /q)

[0047] where SQRT (kT×μ_(n)×τ_(n)/q) is the square root of the bracketedexpression where:

[0048] k is the Boltzmann constant,

[0049] T is the temperature of the semiconductor,

[0050] μ_(n) is the electron mobility in p-type material,

[0051] τ_(n) is the electron lifetime in p-type material,

[0052] q is the electric charge.

[0053] The mobility, μ_(n), of electrons in the p-type material isrelated to the doping concentration of the semiconductor material: thelower the doping the higher the mobility, μ_(n). The electron lifetime,τ_(n), in non-depleted p-type material is directly related to therecombination rate of the electrons with holes. This parameter too isdefined by the p-type semiconductor material. As can be seen from theformula, the diffusion length, L_(n), depends heavily on,the choice ofthe material of the p-type semiconductor region. Therefore, by choosingthe appropriate p-type semiconductor material of the p-typesemiconductor region or emitter tip, the diffusion length, L_(n), can bevaried over a wide range. Typically, the diffusion length, L_(n), ofp-type material used for p-type emitter tips varies between a micrometerup to hundreds of micrometers.

[0054] In the model of the bipolar npn-transistor, the n-typesemiconductor region corresponds to the emitter, the p-typesemiconductor region to the base and the extracting electrode to thecollector. Using the analogy, the electron beam device according to theinvention preferably is operated in the “saturation mode” where both thepn-diode (emitter diode) and the extracting electrode (collector diode)are biased in a forward direction. The first voltage V1 between p-typesemiconductor region and extracting electrode preferably is so high thatthe electron emission current depends only slightly or not at all onchanges of the first voltage V1. In the saturation mode, the emissioncurrent therefore shows improved emission current stability even whenthe electric field at the emitter tip changes due to changes of shape orsurface states of the emitter tip during operation. The presentinvention therefore overcomes the notorious problem of large emissioncurrent instabilities of field emission cathodes.

[0055] Preferably, the first voltage V1 between the extracting electrodeand the first electric contact is provided. The size of the positivefirst voltage V1 depends on the geometry of the extracting electrode andthe emitter tip. Among the most important parameters are the emitter tipheight, H, from the base of the emitter tip to the apex of the emittertip, the radius of the apex of the emitter tip, the length of theemitter tip and the material of the emitter tip. For an emitter tip madeof silicon, the necessary field strength for significant electronemission is preferably above 10⁹ V/m. In this case the thickness of thepotential barrier, T, through which electrons have to tunnel forelectron emission is smaller than a few tens of nanometers. When theextracting electrode is positioned near the emitter tip as close asroughly 500 nm to 2 μm the positive first voltage may be as low as e.g.20 to 200 V.

[0056] Preferably a forward biasing second voltage V2 across thepn-diode of the p-type semiconductor region and the n-type semiconductorregion is provided. The second voltage V2 controls the pn-diode current,i.e. the electron current injected into the p-type semiconductor region.Preferably, the second voltage V2 is very stable to generate a stableelectron emission current, since in the saturation mode, the injectedelectron current determines the electron emission current. For fieldemission cathodes made of silicon material, this second voltage ispreferably in the range between −1 V to 1V, which is the range to switchthe pn-diode on or off. For pn-diodes made of other semiconductormaterials the voltages to switch on or switch off may be somewhatdifferent.

[0057] Preferably, the field emission cathode is integrated with asemiconductor substrate. Preferably, the field emission cathode isintegrated on a semiconductor substrate. The integration of a p-typesemiconductor region with emitter tip and an n-type semiconductor regionon a semiconductor is a well-known technique in the field ofsemiconductor microprocessing. It allows for an easy and cost-effectivemanufacturing of field emission cathodes with high geometricalprecision. Preferably the semiconductor substrate is of the samematerial as the p-type semiconductor region and emitter tip. Preferablythe semiconductor substrate is silicon because of the large availabilityof the material and the large diversity of processing techniques.However, the present invention also applies to all other semiconductormaterials which can be p-doped and n-doped, have a sufficient diffusionlength and can be structured in the required shape.

[0058] Preferably, also the extracting electrode is integrated onto thesemiconductor substrate. By using microprocessing techniques for theintegration of the extracting electrode onto the semiconductor substrateit is possible to position the extracting electrode as close as amicrometer or even a fraction of a micrometer to the emitter tip. Thisin turn allows extremely high electric fields at the emitter tip to begenerated at a moderate first voltage value. Furthermore, the design offield emission cathodes with an integrated extracting electrode is morecompact and more precise.

[0059] Preferably, the extracting electrode has an opening through whichemitted electrons of the electron beam can pass to an anode. In thisembodiment, the extracting electrode serves to extract electrons fromthe emitter tip while the anode serves to direct the emitted electronstowards some target. This way the electron emission rate control can beperformed independently from the electron beam guidance control. Theseparation of the two procedures is important for electron beam deviceslike e.g. electron microscopes or electron beam pattern generators wherethe electron beam is to be directed in changing directions while theelectron emission current value has to remain constant.

[0060] The design with an extracting electrode having an opening isstrongly preferred for field emission cathodes with an integratedextracting electrode. There, the extracting electrode is so close to theemitter that the emitted electrons might not be of any use if they couldnot pass through a hole of the extracting electrode to an anode.

[0061] Preferably, the electron beam device comprises focussingcomponents to direct and focus the beam. Focussing components may bemagnetic lenses, deflection coils, anodes and other devices useful forelectron beam deflection and beam focussing. For precision devices likeelectron microscopes and electron beam pattern generators it isimportant to adjust the electron emission current without changing theelectrostatic fields in the electron beam region. Since in thesaturation mode the electron emission current can be adjusted withoutchanging the first voltage V1 between extracting electrode and emittertip possible electric field interference due to a fluctuating extractingelectrode potential into the electrical field of the electron beamoptics cannot occur.

[0062] Preferably, the emitter tip is coated with coating material. Thecoating material may be on the emitter tip because of the manufacturingprocedure or for better emitter tip stability. The coating material mayalso serve to reduce the leakage current due to surface generationcenters at the surface of the emitter tip. The leakage current generateselectrons which can be emitted without having traveled through thep-type semiconductor region. The leakage current therefore circumventsthe electron emission control through the second voltage V2 across thepn-diode. Therefore, it is in the interest of good electron emissioncurrent control to minimize the leakage current.

[0063] In order to reduce the surface generation centers the coatingmaterial preferably is a passivation layer, e.g. silicon oxide for anemitter made of silicon. On the other hand, the layer of the coatingmaterial must be thin enough to not impede electron emission through atoo high potential barrier thickness, T. For that reason, the thicknessof the coating material at the apex of the emitter tip is preferably notthicker than tens of nanometers.

[0064] The invention according to claim 12 provides an electron beamdevice comprising an array of field emission cathodes with an array ofextraction electrodes. An array of field emission cathodes allows anarray of electron beams to be generated. Electron beam arrays are usefulin many applications. For flat panel displays they are a preconditionfor generating a two-dimensional image on a screen. In applications likeelectron microscopy or electron beam pattern generators they allow forparallel inspection or parallel processing for improving e.g. theproduction throughput. However, these are only few of the many otherapplications for which arrays of field emission cathodes are useful.

[0065] Preferably the array of field emission cathodes is integratedonto a substrate, preferably a semiconductor substrate. The integrationof arrays of field emission cathodes onto a substrate allows for the useof microprocessing manufacturing technology. With the use ofmicroprocessing manufacturing technology arrays of field emissioncathodes can be fabricated with high geometric and electronic precision,which helps to make the array of field emission cathodes homogeneous infunctionality and positioning. Also, microprocessing manufacturingtechnology makes it possible to fabricate arrays with up to thousands ormillions of field emission cathode on a silicon-size chip. The distancebetween neighboring field emission cathodes of such arrays may be in therange between millimeters down to less than one micrometer.

[0066] Preferably, the extracting electrodes of the array of extractingelectrodes are electrically connected with each other. Preferably, theyare connected with each other by low ohmic connections. Preferably, theextracting electrodes are at the same electric potential, which allowsall extracting electrodes to be connected to only one external electriccontact. This is a significant advantage compared to the situation wherelarge arrays of extracting electrodes have to be connected individually.Thousands or even millions of conducting lines or contacts can be savedin this way. For many applications it is sufficient to connect theextracting electrodes of the array of extracting electrodes in such away that the array of extracting electrodes is simply one conductingplate or one conducting layer.

[0067] It is one of the advantages of the present invention that in thesaturation mode with the extracting electrodes at the same potential theelectron beam currents can still be adjusted individually. In thesaturation mode, the emission current depends only slightly or not atall on the first voltage V1. This implies that an array of many fieldemission cathodes, which to some degree all have individual geometricshapes, can be operated with the same first voltage V1 while stilldelivering a homogenous array of electron beams.

[0068] Preferably, also the n-type semiconductor regions of the array offield emission cathodes are electrically connected with each other.Preferably, the electrical connection is low-ohmic in order to have then-type semiconductor regions at the same potential. This embodimentsaves the many conducting lines that otherwise would be necessary tocontact the n-type semiconductor regions individually. Preferably, theelectrical connection is made by having the n-type semiconductor regionstouch each other. Preferably, the n-type semiconductor regions toucheach other in such a way that the many n-type semiconductor regions makeup one n-type semiconductor region. In this embodiment, the structuringof the n-type semiconductor region on the substrate can be saved toreduce costs.

[0069] Preferably, the p-type semiconductor regions of the array offield emission cathodes are electrically connected with each other.Preferably, -the electrical connections are low-ohmic in order to havethe p-type semiconductor regions at the same potential. This embodimentsaves the many conducting lines that otherwise would be necessary tocontact the p-type semiconductor regions individually. Preferably, theelectrical connections are made by having the p-type semiconductorregions touch each other. Preferably the p-type semiconductor regionstouch each other in such a way that the many p-type semiconductorregions make up one p-type semiconductor region. In this embodiment, thestructuring of the p-type semiconductor region on the substrate can besaved to reduce costs.

[0070] It depends on the application which of the three electricconnections, extracting electrode, p-type semiconductor region or n-typesemiconductor region, should be common to all field emission cathodes ofthe array of field emission cathodes. If all three connections each arecommon to all field emission cathodes of the array of field emissioncathodes, only two voltage sources are needed to bias an arbitrarilylarge array of field emission cathodes; however in this case theelectron beam currents are not adjustable individually.

[0071] With the extracting electrodes of all field emission cathodes ata common potential and the p-type semiconductor regions of all fieldemission cathodes at a common potential, the n-type semiconductorregions are preferably connected to individual voltage supplies. In thiscase the electron beam currents can be adjusted individually by means ofadjusting the second voltage V2, which for many applications isadvantageous. Another advantage of this embodiment is that theelectrical potential in the electron beam region is undisturbed when theelectrical beam currents are adjusted, since a change of n-typesemiconductor region potentials does not affect the external electricfield of the region between emitter tips and extracting electrodes.

[0072] The method used to generate at least one electron beam currentwith the electron beam devices according to the invention comprises thesteps of applying a positive first voltage V1 to the extractingelectrode with respect to the p-type semiconductor region, and applyinga second voltage V2 to the pn-diode junction formed by the p-typesemiconductor region with the n-type semiconductor region.

[0073] Preferably, the second voltage V2 is a voltage forward biasingthe pn-diode junction. In this case electrons from the n-typesemiconductor region pass the pn-diode junction to enter into the p-typesemiconductor region, from where they can travel to the apex of theemitter tip for electron emission into free space.

[0074] Preferably, the free space between emitter and extractingelectrode is operated at a vacuum better than 10⁻⁶ mbar and preferablybetter than 10⁻⁹ mbar. A good vacuum value reduces the collision ratebetween the electron beam and residue gas which can destroy the electronbeam on its way to its target. A good vacuum also impedes chemicalreactions at the emitter tip which can deform the shape or surface stateof the sharp tip. If too strong, such changes can deteriorate theoperational lifetime of a field emission cathode.

[0075] The present invention however reduces the emission currentinstabilities due to bad vacuum, since in the saturation mode theelectron beam current is less sensitive to variations of the electricfield at the emitter tip. The vacuum may be generated each time that theelectron beam device is put into operation, e.g. by a vacuum pump;however the vacuum may also be permanent, i.e. the volume betweenextracting electrode and emitter tip is evacuated once and sealed in avacuum proof cartridge.

[0076] Preferably the positive first voltage V1 is increased to a levelwhere the electron beam current has reached a saturation current value.Like the saturation region of a bipolar npn-transistor, a saturationcurrent value of a field emission cathode is reached when the firstvoltage V1 is above the saturation threshold, i.e. above a voltage atwhich the current gain per voltage has decreased significantly.

[0077] The invention according to claim 32 further provides a fieldemission cathode connected with an emitter tip made of p-typesemiconductor material, whereby essentially all electrons entering theemitter tip flow through the p-type semiconductor region. The p-typesemiconductor material of the emitter tip makes sure that there are onlyfew or no free electrons available within the emitter tip (minoritycarriers). The fact that essentially all electrons entering the emittertip flow through the p-type semiconductor region implies that there areno other pathways for electrons to flow to the emitter tip than byflowing through the p-type semiconductor region. This has the advantagethat the electron current flowing to the emitter tip can be fullycontrolled by the second voltage V2 between the p-type semiconductorregion and the n-type semiconductor region.

[0078] In another preferred embodiment of the invention, the pn-diodecan be a tunnel diode. A tunnel diode is a diode with the p-type regionand the n-type region so heavily doped that in thermal equilibrium theFermi-level of the p-type material is within the energy region of theconducting band of the n-type region. This feature produces thewell-known current-voltage curves of tunnel diodes with a region withnegative differential resistance. With the second voltage V2 in theregion with negative differential resistance, it is possible to controlthe electron beam device in such a way that an increase of the secondvoltage V2 reduces the electron emission current.

[0079] In another preferred embodiment of the invention, the pn-diodecan be the collector diode of a bipolar pnp-transistor. In this case thep-type semiconductor region, the n-type semiconductor region and asecond p-type semiconductor region form a bipolar pnp-transistor, wherethe p-type semiconductor region is the collector, the n-typesemiconductor region the base and the second p-type semiconductor regionthe emitter. Preferably the electron current injected into the p-typesemiconductor region is determined by the voltage between the emitterand the base. In this case the electron emission current can becontrolled by the emitter-base voltage independent of the voltage of thep-type semiconductor region, provided that the first voltage V1 is insaturation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0080] Some of the above indicated and other more detailed aspects ofthe invention will be described in the following description andpartially illustrated with reference to the figures. Therein:

[0081]FIG. 1a-b show schematically a first embodiment of a fieldemission cathode according to the invention with and without externalelectric field.

[0082]FIG. 2a-b show schematically a second embodiment of a fieldemission cathode according to the invention with and without externalelectric field.

[0083]FIG. 3a-b show schematically a third embodiment of a fieldemission cathode according to the invention with and without externalelectric field.

[0084]FIG. 4a-b show schematically a fourth embodiment of a fieldemission cathode according to the invention with and without externalelectric field.

[0085]FIG. 5a-b show schematically a fifth embodiment of a fieldemission cathode according to the invention with and without externalelectric field.

[0086]FIG. 6a-c show schematically various embodiments of electron beamdevices according to the invention with one field emission cathode andone extracting electrode.

[0087]FIG. 7a-c show schematically a method to generate an electron beamwith an electron beam device according to the invention, whereby theemitter tip is p-type material.

[0088]FIG. 8a-c show schematically a method to generate an electron beamwith an electron beam device according to the invention, whereby aportion of the emitter tip is n-type material.

[0089]FIG. 9 shows a field of current-voltage curves of an electron beamdevice according to the invention.

[0090]FIG. 10a-d show schematically various embodiments of electron beamdevices according to the invention with arrays of field emissioncathodes and at least one extracting electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0091] In FIG. 1a-b, FIG. 2a-b, FIG. 3a-b, FIG. 4a-b and FIG. 5a-b, fiveembodiments of field emission cathodes according to the invention areshown schematically. FIGS. 1a, 2 a, 3 a, 4 a and 5 a refer to theembodiments without external electric field while FIGS. 1b, 2 b, 3 b, 4b and 5 b refer to the same embodiments with an external electric fieldswitched on. The figures with the external electric field switched onrefer to the situation when the strength of the external electric fieldat the apex of the emitter tips is high enough that the field emissioncathodes are operated in the saturation mode, i.e. in the mode where theelectron emission current is limited by the electron injection into thep-type semiconductor region.

[0092] Preferably, the field emission cathodes of the preferredembodiments are made on a silicon substrate, because the processtechnology for generating the desired electrical and physical structureon silicon is well known. However substrates with other semiconductormaterials would work as well.

[0093] In FIG. 1a the field emission cathode 3 without external electricfield is shown. It comprises an n-type semiconductor region 11 with ap-type semiconductor region 7 forming a pn-diode junction 13 with apn-diode depletion zone 14. The p-type semiconductor region 7 is furtherconnected to an emitter tip 9 which points into fee space 27. Theemitter tip 9 in FIGS. 1a and 1 b is made of p-type doped semiconductormaterial. The non-depleted p-type portion of the emitter tip 9 and thenon-depleted p-type portion of the p-type semiconductor region 7therefore form one non-depleted p-type portion 18. Therefore the p-typesemiconductor region 7 is in ohmic contact with the emitter tip 9, i.e.the potential of the emitter tip 9 is controlled by the voltage of thep-type semiconductor region 7.

[0094] The emitter tip 9 has a height H, which is the distance from theemitter tip base 16 to the apex of the emitter tip 10. The emitter tipbase 16 is the line that separates the semiconductor substrate 37 fromthe emitter tip 9. The p-type semiconductor region 7 is connected to theemitter tip 9 in such a way that an electron current entering theemitter tip 9 must flow through the non-depleted p-type portion 18.Without external electric field, the minimum thickness that electronshave to travel through non-depleted p-type material, i.e. the minimumnon-depleted p-type distance D reaches from the pn-diode junction 13 tothe apex 10 of the emitter tip 9.

[0095] The p-type semiconductor region 7 further comprises a firstelectric contact 15 in order to be able to apply an external voltage tothe p-type semiconductor region 7. Preferably the first electric contact15 is an ohmic contact. Preferably, the first electric contact 15comprises a conducting layer element that is connected to a conductingline making contact to a voltage source. In order to have a low contactresistance the p-type semiconductor region 7 is preferably highly dopedin the region where the conducting layer element makes contact with thep-type semiconductor region 7. The vertical extension of the p-typesemiconductor region 7 preferably is small to minimize the minimumnon-depleted p-type distance D. Preferably, the vertical extension ofthe p-type semiconductor region 7 is below one micrometer.

[0096] The n-type semiconductor region 11 comprises a second electriccontact 17 in order to be able to apply an external voltage to then-type semiconductor region 11. Preferably the second electric contact17 is an ohmic contact. Preferably, the second electric contact 17, too,comprises a conducting layer element that is connected to a conductingline making contact to a voltage source. In order to have a low contactresistance the n-type semiconductor region 11 is preferably highly dopedin the region where the conducting layer element makes contact with then-type semiconductor region 11.

[0097] There are several possibilities to manufacture a structure likein FIG. 1a. To give an example, an n-type semiconductor substrate may beselectively etched to form a sharp tip with an apex radius of a fewnanometers and a length of a few micrometers. The sharp tip -serves asemitter tip 9 which emits electrons preferably at the apex. Methods toform such sharp tips with microprocessing techniques from asemiconductor material are known in the art. After forming the sharptip, the r-type semiconductor substrate is selectively doped with p-typedoping material in the region of the sharp tip to form the p-typesemiconductor region 7 and the p-type emitter tip 9. The lateralextension of the p-type semiconductor region 7 preferably is largeenough that the emitter tip base 16 is fully contained within thesurface of the p-type semiconductor region 7, and that a first electriccontact 15 can be applied to the p-type semiconductor region 7.

[0098] After generating the p-type semiconductor region 7 the first andsecond electric contacts, 15 and 17, are generated on the p-typesemiconductor region 7 and n-type semiconductor region 11. Both electriccontacts are preferably realized as ohmic contacts with a lowresistance. The first electric contact 15 may consist e.g. of analuminum layer element which is in contact with the p-type semiconductorregion 7, whereby the p-type semiconductor region 7 is preferably highlyp-doped in the region where the contact is made.

[0099] Analogously, the second electric contact 17 may consist e.g. ofan aluminum layer element which is in contact with the n-typesemiconductor region 11, whereby the n-type semiconductor region ispreferably highly n-doped in the region where the contact is made.Details of making ohmic contacts on semiconductors are well-known in theart and are not further described.

[0100]FIG. 1b shows the same field emission cathode 3 like in FIG. 1a,however with a positive external electric field 28 switched on. Withincreasing external electric field strength leakage current generated inthe emitter tip 9 increasingly gets emitted into free space 27. Theregion around the apex 10 of the emitter tip 9 therefore is increasinglydepleted of free electrons and free holes to form a depleted p-typeemitter region 20.

[0101] With an increasing depleted p-type emitter region 20 the minimumnon-depleted p-type distance D becomes shorter. A shorter non-depletedp-type distance D in turn increases the electron transport efficiencythrough the non-depleted p-type portion 18. If the external electricfield 28 at the surface of the emitter tip 9 is larger than 10⁶ V/cm theelectrons arriving at the surface can be emitted into free space 27.

[0102] It is preferred to make the minimum non-depleted p-type distanceD as short as possible. Preferably, the minimum non-depleted p-typedistance D is shorter than the diffusion length, L_(n), and preferably10 times shorter than the diffusion length, L_(n). To comply with thiscondition, the diffusion length L_(n), is preferably chosen to be aslong as possible. This can be achieved by having the p-type materiallowly doped, by processing the p-type material in a way that it has onlyfew recombination centers, or by increasing the temperature of the fieldemission cathode 3.

[0103] The size of the depleted p-type emitter region 20 might besmaller or even disappear when the leakage current generation in theemitter tip 9 is significant. If the leakage current generation is ofthe range or even larger than the electron emission current, theelectrons may shield the emitter tip 9 from the external electric field28, which in turn reduces the size of the depleted p-type emitter region20. In order that the control of the electron emission current by thesecond voltage, V2, is not circumvented by leakage current generation,it is preferred to process the emitter tip in a way that the density ofleakage current centers density in the emitter tip 9 is minimized.

[0104] In FIG. 2a another field emission cathode 3 without externalelectric field is shown. In FIG. 2a the doping levels within the p-typesemiconductor region 7 are varied. The two p⁺-type semiconductor regionsare highly doped to provide a first electric contact 15 with lowresistance to the p-type semiconductor region 7. The high doping levelsalso provide a low ohmic connection to the emitter tip 9 to keep thep-type region at a well defined electric potential. In this embodiment,the p⁺-type semiconductor regions have a doping concentration preferablylarger than 10¹⁶ l/cm³, preferably larger than 10¹⁸ l/cm³ and even morepreferably larger than 10¹⁹ l/cm³.

[0105] The two p-type semiconductor regions in the p-type semiconductorregion 7 and in the emitter tip 9 are lowly doped to provide a largediffusion length, L_(n), in order to provide a large electron transportefficiency. A high electron transport efficiency allows for theoperation of the field emission cathode 3 with a low pn-diode currentinjection at a given electron emission current. A low injection currentreduces the noise fluctuations of the electron emission current,increases the stability of the field emission cathode operation andreduces heating. Heating might be an important issue for large arrays offield emission cathodes. The low p-type doping also reduces the minimumnon-depleted p-type distance D since the pn-diode depletion zone 14 isextended. In this embodiment, the p-type semiconductor regions have adoping concentration preferably smaller than 10¹⁶ l/cm³, preferablysmaller than 10¹⁵ l/cm³ and even more preferably smaller than 10¹⁴l/cm³.

[0106]FIG. 2b shows the field emission cathode 3 like in FIG. 2a withthe external electric field 28 switched on. Because of the low doping inthe emitter tip 9, the depleted p-type emitter region 20 is larger thanin FIG. 1b. This in turn reduces the minimum non-depleted p-typedistance D to improve the electron transport efficiency.

[0107] At the same time, the highly doped p⁺-regions of p-typesemiconductor region 7 help to prevent the depleted p-type emitterregion 2o from growing through the non-depleted p-type portion 18 to thepoint that the depleted p-type emitter region 20 touches the pn-diodedepletion zone 14. In this case the minimum non-depleted p-type distanceD would be zero. This can happen when the p-doping is very low, theexternal electric field 28 very high or the emitter tip height, H, verysmall. With the minimum non-depleted p-type distance D at zero, theelectrons injected into the p-type semiconductor region could passthrough the p-type semiconductor region without recombination. However,there would be no electron emission current control through the secondvoltage V2 anymore.

[0108] The field emission cathode 3 as shown in FIGS. 2a and b ispreferably used for applications where a high electron emission current,e.g. larger than 10 nA, preferably larger than 100 nA is needed. In thiscase, a high transport efficiency is important. However, it comes as adisadvantage that the low doping level of the emitter tip 9 leads to alarge volume of the depleted p-type emitter region 20 when the externalelectric field 28 is applied. The large volume of the depleted p-typeemitter region 20 causes a high leakage current which adds to theemission current. Since the leakage current cannot be controlled by thesecond voltage V2, it is preferred that the leakage current generated inthe emitter tip 9 is at least an order of magnitude smaller than theelectron current entering the pn-diode junction 13.

[0109] In addition, the low doping level of the p-type emitter tip 9also leads to a larger resistance of the emitter tip. While a largeemitter resistance can serve to stabilize the emission current in aself-regulating fashion, it can also be too high if a large emissioncurrent is desired for a given application.

[0110]FIGS. 3a and 3 b show an embodiment of the invention like FIGS. 2aand 2 b with the difference that the emitter tip 9 is a highly dopedp⁺-type region. The high p-type doping provides a reduced volume of thedepleted p-type emitter region 20 when the external field 28 is switchedon, compared to FIG. 2b. The smaller volume of the depleted p-typeemitter region 20 in turn leads to a reduced leakage current. So that alarge fraction of the emitted current can be controlled by the secondvoltage V2, it is important that the leakage current is significantlysmaller than the total emission current. Therefore a high p-type dopingof the emitter tip 9 is important when the total emission current of thefield emission cathode 3 is small, i.e. smaller than 50 nA andpreferably smaller than 10 nA. As a disadvantage, the high doping of theemitter tip 9 increases the minimum non-depleted p-type distance D,which in turn reduces the electron transport efficiency. Preferably, thep⁺-type doping of the emitter tip is larger than 10¹⁶ l/cm³, preferablylarger than 10¹⁸ l/cm³ and even more preferably larger than 10¹⁹ l/cm³.

[0111] In FIG. 4a a fourth embodiment of a field emission cathode 3without external electric field is shown, which differs from FIG. 1a,FIG. 2a or FIG. 3a in that the emitter tip 9 comprises two regions. Thefirst emitter tip region is the emitter p-type region 9 a, which isconnected to the p-type semiconductor region 7. The second region is theemitter n-type region 9 b which is not connected to the p-typesemiconductor region 7 but comprises the apex 10 of the emitter tip 9.With this design the minimum non-depleted p-type distance D can be mademuch shorter than with an emitter tip 9 made of p-type material only, toincrease the electron transport efficiency. However the upper part ofthe emitter tip 9, i.e. the emitter n-type region 9 b, is not in ohmicconnection with the p-type semiconductor region 7 because of the secondpn-diode junction 81 between emitter p-type region 9a and emitter n-typeregion 9 b. As a consequence the electric potential of the emittern-type region 9 b cannot be electrically controlled by a second voltageV2, i.e. the voltage between the first and second electric contact, 15and 17. It also depends on the external field 28 at the surface of theemitter tip 9. This implies, that the energy of the electrons beingemitted from the emitter tip 9 depends not only on the second voltage V2but also on the first voltage V1. The second pn-diode junction depletionzone 80 due to the second pn-diode junction 81 is not shown in FIG. 4a.

[0112]FIG. 4b shows the field emission cathode 3 of FIG. 4a with theexternal electric field 28 switched on. In this case the minimumnon-depleted p-type distance D decreases due to the expanding secondpn-diode junction depletion zone 80. In the saturation mode, i.e. whenthe potential barrier at the surface of the emitter tip is small enoughthat the electron emission current is determined by the electroninjection into the p-type semiconductor region, the emitter n-typeregion 9 b is fully depleted.

[0113]FIG. 5a shows a field emission cathode 3 like in FIG. 1a with theemitter tip 10 coated with a coating material 8. In one preferredembodiment the coating material 8 serves to reduce the density ofleakage current generation centers at the surface of the emitter tip 9.A low leakage current is desirable for two reasons; firstly, the leakagecurrent adds to the electron emission current but cannot be directlycontrolled by voltage across the pn-diode junction 13. In particular,the electron emission current cannot be switched off with the permanentleakage current on. Secondly, the external electric field strength atthe emitter tip 28 which is necessary to generate an electron emissioncurrent independent of the strength of the external electric field 28 ishigher (saturation mode). This is because the thickness of the potentialbarrier at the surface of the emitter tip 9 must be smaller in orderthat the electrons from the leakage current can tunnel through withoutsignificant delay.

[0114] If the emitter tip is made of silicon, the coating material ispreferably made of silicon oxide. The layer thickness of the coatingmaterial 8 is low in order to not broaden the emitter tip 9 by too much.Preferably the layer thickness of the coating material 8 at the emittertip 9 is below 100 nm and preferably below 10 nm in order to notdiminish the external electric field in the region of the apex 10 of theemitter tip. Preferably, the apex 10 is not coated with coating material8 in order to keep the potential barrier at the surface of the emittertip 9 at the apex 10 small.

[0115]FIG. 5b shows the field emission cathode 3 of FIG. 5a with theexternal electric field 28 switched on. The coating material 8 on theemitter tip 8 keeps the leakage generation current small. Therefore, theexternal electric field 28 at the emitter tip 9 which is necessary tooperate the field emission cathode 3 in the saturation mode is lower.

[0116] The embodiments of the invention as shown in FIG. 1a to FIG. 5arepresent only a few examples of many other possible modifications ofthe invention. In particular, the doping profiles or the geometriclayout of the emitter tip 9, the p-type semiconductor region 7 or then-type semiconductor region 11 may be varied in many different featuresin order to optimize the device performance to a given application.However, from reading the disclosure, the modifications and variationswill be apparent to persons skilled in the art. Such modifications andvariations may also involve equivalent features and other features whichare already known in the art and which may be used instead of or inaddition to features already disclosed herein.

[0117]FIG. 6a shows a first embodiment of an electron beam device 1according to the invention. The electron beam device 1 comprises a fieldemission cathode 3 and one extracting electrode 5. The field emissioncathode 3 can be any of those described in the description. The fieldemission cathode 3 shown in FIG. 6a equals the field emission cathode 3shown in FIG. 1a. The field emission cathode 3 comprises a p-typesemiconductor region 7 with an emitter tip 9 and an n-type semiconductorregion 11. The n-type semiconductor region 11 and the p-typesemiconductor region 7 together form the pn-diode junction 13.Furthermore, a first voltage source 21 is shown which is able togenerate a positive first voltage V1 with respect to the p-typesemiconductor region 7, and a second voltage source 23 is shown which isable to generate a forward biasing voltage V2 across the pn-diodejunction 13.

[0118] By applying a sufficiently large first voltage V1 between theextracting electrode 5 and the p-type semiconductor region 7, a highexternal electric field 28 is generated at the emitter tip 9. Themaximum field strength of the external electric field is generated atthe apex 10 of the emitter tip 9. Accordingly, free electrons arepreferably emitted at the apex 10 of the emitter tip 9. If the firstvoltage V1 is high enough that the potential barrier at the surface ofthe emitter tip 9 is thin enough that all electrons reaching the apex 10of the emitter tip 9 can tunnel through without delay, the externalelectric field penetrates into the emitter tip region forming thedepleted p-type emitter region 20. The electron beam device 1 is thensaid to operate in the saturation mode.

[0119] The extracting electrode 5 serves to apply a high electric fieldto the emitter tip 9, and in particular to the apex 10 of the emittertip 9. The minimum field necessary to emit electrons in the range ofnanoamperes from the emitter tip 9 is about 10⁸ V/m to 10⁹ V/m. Toachieve such high electric fields at the apex 10 of the emitter tip 9 atreasonable voltages, the apex radius has to be very small (e.g. in thenanometer scale) and the ratio of the length of the emitter tip to theapex radius rather large (a few hundreds). With the extracting electrode5 as close as about a micrometer to the emitter tip apex 10 it ispossible to achieve significant emission beam currents with voltages aslow as 20 to 100 V. If higher voltages V1 are allowed between emittertip 9 and extracting electrode 5 then of course the extracting electrode5 can be positioned further away from the emitter tip. The first voltageV1 between the extracting electrode 5 and the first electric contact 15is supplied by the first voltage source 21.

[0120] The second voltage source V2 provides the voltage between thefirst electric contact 15 and the second electric contact 17, which inturn supplies the voltage to operate the pn-diode consisting of thep-type semiconductor region 7 and the n-type semiconductor region 11.When the voltage at the n-type semiconductor region 11 is more positivethan the voltage at the p-type semiconductor region 7, the pn-diode isbiased in a forward direction. This implies that electrons from then-type semiconductor region 11 cross the pn-diode junction 13 into thenon-depleted p-type portion 18 where they travel randomly around untilthey either reach the depleted p-type emitter region 20, the surface ofthe apex 10 or they recombine with the abundantly available holes in thep-type semiconductor region 7. Preferably, the electron emission current19 is controlled through second voltage V2 by keeping first voltage V1constant. This way, the electric field in free space 27 is not affectedby switching or changing the electron emission current 19.

[0121] Those electrons which reach the depleted p-type emitter region 20see the external electric field generated by the extracting electrode 5,and drift towards the apex 10 of the emitter tip 9. At the surface ofthe apex 10 the potential barrier is preferably so thin that theelectrons can tunnel through it. Once emitted from the surface of theemitter tip into free space 27 the electrons drift to the extractingelectrode 5. Since the free space 27 between emitter tip 9 andextracting electrode 5 is preferably evacuated the electrons drift tothe extracting electrode 5 with few or no collisions with residual gasatoms or molecules.

[0122] The conducting lines shown in FIG. 6a may be simple cables, butthey may also be conducting lines printed on a circuit board orstructured on a substrate.

[0123] In FIG. 6b an electron beam device 1 is shown with a fieldemission cathode 3 identical to the one shown in FIG. 1b; however theextracting electrode 5 is different in that the extracting electrode 5has an opening 6 through which the electron beam 19 can pass whenanother anode 32 is there to attract the electrons. In FIG. 6b the anodeis at a voltage given by the third voltage source 30 which preferablymakes the potential of the anode more positive than the potential of theextracting electrode 5. The opening 6 in the extracting electrode 5 hasthe advantage that the extracting electrode 5 can be positioned veryclosely to the emitter tip while the electron beam 19 can still go somedistance to perform functions like. e.g. scanning a specimen for anelectron microscope, scanning a wafer surface on an electron beampattern generator or exciting light emission on a phosphorus screen.Further, the extracting electrode 5 can be positioned as close as a fewmicrometers or even closer to the emitter tip 9. For such a design, itis possible to integrate the extracting electrode 5 by usingmicroprocessing techniques, which allows for a compact and costeffective manufacturing. In addition, such a close distance betweenextracting electrode 5 and emitter tip 9 makes it possible to achieve asufficient electric field strength for electron emission at the emittertip 9 at a moderate first voltage V1, e.g. below 100 V. The use of lowvoltages eliminates the many known problems that arise with the use ofhigh voltages like e.g. above 1 kV or above 10 kV.

[0124] In FIG. 6c an electron beam device 1 like in FIG. 6b is shown.The only difference is the introduction of focussing components 34 intothe path of the electron beam 19. The focussing components 34 in FIG. 6crepresent any electric or magnetic device or a combination of thosedevices which focus or direct the electron beam 19 to a position.Focussing components 34 are used e.g. for electron microscopes orelectron beam pattern generators. The electron beam device 1 as shown inFIG. 6c offers high stability of the electron beam current since theemission current is preferably controlled by the pn-diode current whichis controlled by the second voltage V2. In addition, even when anadjustment of the electron beam current value is necessary, thecorrection is preferably done by adjusting the second voltage V2 acrossthe pn-diode junction 13, which usually only has to be changed by lessthan a volt. Such a change is hardly sensed by the focussing components34. As a result, with a device as shown in FIG. 6c it is possible tocontrol the electron beam current without significant interference withelectrical or magnetic fields determining the path of the electron beam19. This is a major advantage over electron beam devices where theemitter current value is controlled by the voltage between the emittertip and the extracting electrode. Those voltages tend to interfere withthe electric field that controls the path of the electron beam.

[0125] In FIGS. 7a to 7 c position-energy plots are shown whichschematically illustrate the underlying physical model as they arethought to be valid for the electron emission of electron beam devices,using field emission cathodes like in FIG. 1a. The physical model shownin FIGS. 7a to 7 c serves as an attempt to explain the invention,however FIGS. 7a to 7 c are not meant to describe the devices accordingto the invention to the full extent.

[0126] The horizontal axis X represents the positions along the axis ofan emitter tip 9 from the n-type semiconductor region 11 to the apex ofthe emitter tip 10 further to the extracting electrode 5. The verticaldirection meanwhile represents the electron energy levels with theFermi-energy 60, of the lower edge of the conducting band 62, of theupper edge of the valence band 63 and the vacuum energy level 61 thattogether define the emission current of the electron beam deviceaccording to the invention.

[0127] On the left side of the position-energy plot there is the n-typesemiconductor region 11 which reaches to the pn-diode junction 13. Inthe n-type semiconductor region 11 the majority carriers are electronsas indicated by the free electrons 56, drawn on the upper edge of theconducting band line 62. Following the pn-diode junction 13, there isthe p-type semiconductor region 7 which reaches to the position of theapex 10 of the emitter tip 9. In the p-type semiconductor region 7 themajority carriers are holes, as indicated by the free holes 57 drawnbelow the upper edge of the valence band line 63. To the right of theapex position 10 there is free space 27, which preferably is at a goodvacuum, which reaches to the extracting electrode 5. Between apex 10 andextracting electrode 5 is the potential barrier 65 with the potentialbarrier thickness T and a height given by the vacuum energy level 61.

[0128] The lower edge of the conducting band line 62 indicates theenergy sector where free electrons 56 are allowed to move. Electrons arefree electrons when they are above the lower edge of the conducting bandline 62. Without external forces, free electrons 56 tend to move to theposition with the lowest value of the lower edge of the conducting bandline 62. This is why there is an abundance of free electrons 56 in then-type semiconductor region 11 outside the pn-diode depletion zone 14.This region therefore is called non-depleted p-type semiconductorregion.

[0129] The same holds true for holes except for the polarity. The upperedge of the valence band line 63 indicates the energy sector where holes57 are allowed to move. Without external forces holes 57 tend to move tothe position with the highest value of the upper edge of the valenceband line 63. This is why there is an abundance of holes 57 in thep-type semiconductor region 7 outside the pn-diode depletion zone 14.This region therefore is called non-depleted n-type semiconductorregion.

[0130] Finally, the letter E_(g) indicates the gap energy between theupper edge of the valence band line 63 and the lower edge of theconducting band 62. The region between the two bands is called theforbidden band, since in this energy section no electrons or holes areallowed to reside. The gap energy is a constant depending on thesemiconductor material. For silicon, the gap energy, E_(g), is ca. 1.1eV at room temperature.

[0131] The sequence of FIGS. 7a to 7 c schematically shows an example ofthe method to provide an electron beam 19 according to the invention. InFIG. 7a, no external voltages are applied, i.e. the first voltage V1 andthe second voltage V2 are zero. Consequently, the Fermi-energy level 60is at a constant energy. The lower edge of the conducting band 62 andthe upper edge of the valence band 63 arrange themselves around theFermi-energy level 60 according to their doping levels: for the n-typesemiconductor region 11 the Fermi-energy level 60 is closer to theconducting band 62 while in the p-type semiconductor region 7 theFermi-energy level 60 is closer to the valence band 63.

[0132] In the transition region around the pn-diode junction 1 3,conducting band 62 and valence band 63 are bent in order to have acontinuous connection between the left half of the conducting band (orvalence band) and the right half of the conducting band (or valenceband). The bent conducting band line 62 represents a potential barrierwhich prevents the electrons from moving into the p-type semiconductorregion 7 while the bent valence band line 63 represents a potentialbarrier which prevents the oppositely charged holes from moving into then-type semiconductor region 11. The region where conducting band 62 andvalence band 63 deviate from a horizontal line is depleted of freeelectrons, forming the pn-diode depletion zone 14.

[0133] The height of the potential barrier 65 with respect to theconducting band 62 represents the energy that an electron needs in orderto be able to escape into free space, which preferably is a vacuum. Theheight of the potential barrier depends on the semiconductor materialand the doping level. Without externally applied voltage, i.e. atthermal equilibrium, the potential barrier 65 reaches from the apex 10of the emitter tip 9 to the extracting electrode 5. This is usually amacroscopic distance which is too thick for electrons to tunnel through.

[0134]FIG. 7b shows the same position-energy plot as in FIG. 7a with thedifference that an external first voltage V1 is applied between thep-type semiconductor region 7 and an extraction electrode 5, which ispositive with respect to the p-type semiconductor region 7. The firstvoltage V1 generates an electric field in free space 27 which causes thepotential barrier 65 to bend downwards. While bending the potentialbarrier 65 downwards the potential barrier 65 takes on a shape withdecreasing potential barrier thickness, T.

[0135] When first voltage V1 is applied the external electric fieldmoves free holes 57 away from the apex region 10, if no electrons aregenerated within the p-type semiconductor region 7 to shield the p-typesemiconductor region 7 from the external field. In this case theexternal electric field depletes the region around the apex 10 of theemitter tip 9 to form the depleted p-type emitter region 20. As aconsequence, the non-depleted p-type portion with the free holes 57becomes thinner and the minimum non-depleted p-type distance D shrinks.

[0136] When the first voltage V1 has been increased to the level wherethe electric fields strength at the apex 10 of the emitter tip 9 islarger than 10⁸ V/m to 10⁹ V/m the potential barrier thickness, T, is sosmall that free electrons could tunnel through. However with only thefirst voltage V1 on, there may be no free electrons near apex 10 of theemitter tip 9.

[0137] The only free electrons that might be available for electronemission are electrons that have been generated within the non-depletedp-type portion 18 or within the depleted p-type emitter region 20, i.e.the leakage current. When it is desired that the second voltage V2 havefull control over the electron emission current, the leakage currentshould be small, since it cannot be controlled by the second voltage V2.If is allowed to control the electron emission current by both, thefirst and second voltages V1 and V2, the first voltage V1 can also beused to control also the leakage current. However a change of the firstvoltage V1 might cause electrostatic interference with the electron beamoptics of some sorts of electron beam devices.

[0138] In FIG. 7c the same plot as in FIG. 7b is shown with thedifference that in addition to the first voltage V1 the second voltageV2 has been raised from zero to a value that biases the pn-diode at thepn-diode junction 13 in a forward direction. The voltage increase of thesecond voltage V2 raises the levels of the Fermi-energy 60, theconducting band 62 and the valence band 63 in the n-type semiconductorregion 11 with respect to the levels to the p-type semiconductor region7 accordingly. As a result, the potential barrier in the pn-diodedepletion region 14 is reduced such that an electron current flowingfrom the n-type semiconductor region 11 to the p-type semiconductorregion 7, and a hole current flowing from the p-type semiconductorregion 7 to the n-type semiconductor region 11 is initiated. This chargetransport is identical to the charge flow of a forward biased pn-diode.

[0139] The electrons passing through the pn-diode depletion region 14enter into the non-depleted p-type portion 18 where they randomlydiffuse until they either recombine with a hole or reach the depletedp-type emitter region 20. The electric field in depleted p-type emitterregion 20 accelerates the electrons toward the apex 10 of the emittertip 9 where they can tunnel through the potential barrier 65 with thepotential barrier thickness T. Once they have tunneled through thepotential barrier 65 the electrons are emitted into free space 27, whichpreferably is a vacuum, and accelerated toward the extracting electrode5.

[0140] Preferably the first voltage V1 is so high that the electrontunneling rate through the potential barrier 65 is much higher than therate at which electrons pass through the pn-diode depletion region 14.Preferably the first voltage V1 is also so high that the electrontunneling rate through the potential barrier 65 is much higher than therate at which electrons are generated in the non-depleted p-type portion18 or in the depleted p-type emitter region 20. In this case theelectron beam device is operated in the saturation mode. The advantageof operating the electron beam device in the saturation mode is that theemission current is limited by the electron current initiated by theforward biased voltage. This way, small fluctuations in the potentialbarrier level 65 due to chemical or physical changes of the verysensitive apex 10 of the emitter tip 9 have no or only little influenceon the electron emission current. Since it is much easier toelectrically control the potential barrier of a pn-diode junction 13than to control the potential barrier of a vacuum level 61 at thesurface of an apex 10 at very high electric fields, it is much easier tocontrol the current of emitted electrons 65 by means of second voltageV2.

[0141] In addition, the adjustment of the electron emission current 65can be performed with only small changes of the voltage, e.g. within −1Vand +1V, while the same adjustment of the electron emission current 65by the first voltage V1 had to be performed with much higher voltagechanges. Such high voltage changes between the extracting electrode 5and the emitter tip 9 can severely disturb the beam optics of electronbeam devices where the electron beam has to be carefully directed andfocussed, like e.g. with electron microscopes or electron beam patterngenerators.

[0142] In FIGS. 8a to 8 c a second set of position-energy plots is shownwhich schematically illustrates the underlying physical model as theyare thought to be valid for the electron emission of electron beamdevices, using field emission cathodes like in FIG. 4a. The physicalmodel shown in FIGS. 8a to 8 c serves to explain some features of theinvention, however FIGS. 8a to 8 c are not meant to describe the devicesaccording to the invention to the full extent.

[0143] On the left side of the position-energy plot of FIG. 8a there isthe n-type semiconductor region 11 which extends to the pn-diodejunction 13. Following the pn-diode junction 13, there is the p-typesemiconductor region 17 which extends to the emitter tip base 16. Thep-type doping level however continues into the emitter p-type region 9ato the second pn-diode junction 81. Following the second pn-diodedepletion zone 80 to the apex 10 of the emitter tip 9 comes the emittern-type region 9 b. Without an external electric field, electrons are themajority carriers in the n-type semiconductor region 11 and the emittern-type region 9 b, as indicated by the free electrons 56 drawn above thelower edge of the conducting band 62; meanwhile, holes are the majoritycarriers in the non-depleted p-type portion 18. To the right of the apexposition 10 there is free space 27, which preferably is at a goodvacuum, and which reaches to the extracting electrode 5. Between apex 10and extracting electrode 5 is the potential barrier 65 with thepotential barrier thickness T. The potential barrier thickness T isgiven by the distance between apex 10 and extracting electrode 5. Due tothe absence of a first voltage V1 and the thickness T of the potentialbarrier 65 there are no electrons tunneling through the potentialbarrier 65 from the apex of the emitter tip 10 to the extractingelectrode 5.

[0144]FIG. 8b shows the position-energy plot of FIG. 8a with the firstvoltage V1 switched on. The increase of the first voltage V1, which ispositive with respect to the emitter tip, increases the second pn-diodedepletion zone 80. Preferably, the second pn-diode depletion zone 80 isdepleted until it reaches the apex of the emitter tip 10. In this case,the energy of the electrons passing through the potential barrier isessentially given by the first voltage V1. If the second pn-diodedepletion zone 80 is fully depleted up to the apex 10 as shown in FIG.8b, the maximum value of the potential barrier 65 with respect to thevoltage of the p-type semiconductor region 7 may be significantlylowered due to the voltage drop across the second pn-diode depletionzone 80. The voltage drop however can be minimized by either making theemitter n-type region very thin, preferably less than a few tens ofnanometers, or making the n-type doping very low, preferably less than10¹⁴ l/cm². Like in FIG. 7a or 7 b, the large first voltage V1 reducesthe thickness T of the potential barrier 65 to the level that freeelectrons near the apex 10 could easily tunnel through into free space27. This corresponds to an external electric field strength larger than10⁶ V/m. Preferably the first voltage V1 is so high that the emittern-type region 9 b is completely depleted of free electrons 56.

[0145] To be completely depleted, the potential barrier thickness, T,must be thin enough that the electron emission rate is larger than theleakage current in the second pn-diode junction zone 80. Otherwise, thefree electrons 56 generated in the second pn-diode junction zone 80would prevent complete depletion of this region.

[0146]FIG. 8c shows the position-energy plot of FIG. 8b with the secondvoltage V2 is applying a forward biasing voltage across the pn-diodejunction 13 formed by the n-type semiconductor region 11 and the p-typesemiconductor region 7. As a result the free electrons 56 can surmountthe potential barrier of the pn-diode depletion zone 14 to be injectedinto the non-depleted p-type portion 18. There they travel randomlyuntil they reach the depleted emitter n-type region 9 b to drift to theapex 10 of the emitter tip 9. When the rate of electron emission throughthe potential barrier 65 is higher than the electron current injectedinto the non-depleted p-type portion 18, the electron emission currentcan be fully controlled by the second voltage V2 (saturation mode).

[0147] In FIG. 9 a field of current-voltage curves of an electron beamdevice according to the invention is shown. They resemble in many waysthe current-voltage curves of bipolar npn-transistors. In the verticaldirection, the electron emission current, J, is shown; in the horizontaldirection the first voltage, V1, between the extracting electrode S andp-type semiconductor region 7 with emitter tip 9 is shown. The fivecurves, indicated by (1), (2), (3), (4) and (5), correspond tocurrent-voltage curves with increasing second voltage values, whichroughly lie between 0 V and 0.6 V.

[0148] The field of the five current-voltage curves can be divided intothe linear region, L, to the left of the saturation threshold 75, andthe saturated region, S, to the right of the saturation threshold 75. Inthe linear region, L, the electron emission current, J, depends stronglyon the first voltage V1. In this region the electron beam current islimited by the rate at which free electrons tunnel through the vacuumpotential barrier 65. Also slight changes of the shape of the vacuumpotential barrier 65, e.g. by small amounts of polluting chemicals oremitter tip deformation at the apex 10, can significantly change theelectron emission current, J. The linear region , L, therefore isproblematic when a high stability of the electron emission current isneeded.

[0149] In the saturation region, S, the first voltage V1 is so high thatthe potential barrier thickness, T, is reduced to the level thatelectrons can tunnel through the potential barrier at a high rate. Inthe saturation mode the electron tunneling rate is larger than theleakage current and larger than the electron current injected into thenon-depleted p-type portion. In the saturation mode therefore, theelectron beam current is limited by the rate at which free electrons aremade available to the p-type semiconductor region 7 by electroninjection through the pn-diode junction 13. As a consequence, changes ofthe shape of the vacuum potential barrier 65 have only little influenceon the electron emission current. Since it is much easier to control theelectron current through a pn-diode than the current through a vacuumpotential barrier of a tiny apex of an emitter tip, a much higherelectron emission current stability can be achieved.

[0150] In addition, in the saturation mode, the first voltage V1 betweenextracting electrode 5 and p-type semiconductor region 7 can keptconstant since vacuum potential barrier changes due to pollution ordeformation of the apex 10 of the emitter tip 9 have no or only littleeffect on the electron emission rate. A constant first voltage V1 isimportant for electron beam devices with precision beam optics, sincethere even slight changes of the voltages between extracting electrode 5and emitter tip 9 influence the electron beam optics performance. Aconstant first voltage V1 is important for electron beam devices with alarge array of field emission cathodes since they all can be operatedwith the same voltage V1.

[0151] The reason that the current-voltage curves of the electron beamdevice increase even in the saturation region is due to the fact thatwith increasing first voltage V1 the depleted regions in the emitter tiparound the apex increases. An increased depletion zone around the apexof the emitter tip also increases the leakage current which in thesaturation mode adds to the electron emission current.

[0152] In FIGS. 10a to 10 d several embodiments of electron beam deviceswith arrays of field emission cathodes according to the invention areshown.

[0153] In FIG. 10a, a segment of an electron beam device 1 with asegment of an array of field emission cathodes 3 integrated onto asemiconductor substrate 37 is shown. Preferably the semiconductorsubstrate 37 is silicon. In order to provide good electrical insulationbetween the individual n-type semiconductor regions 11 the semiconductorsubstrate 37 is a p-type semiconductor and preferably, its electricpotential is more negative than any of the n-type semiconductor regions11. In FIG. 10a, the electrical potential of the p-type semiconductorsubstrate 37 is provided by a fourth voltage source 31.

[0154] Each field emission cathode 3 of the array of field emissioncathodes comprises an n-type semiconductor region 11 with a secondelectric contact 17, and a p-type semiconductor region 7 comprising anemitter tip 9 and a first electric contact 15. Both electric contacts,15 and 17, preferably are ohmic contacts with a low resistance. Geometryand doping profile of the emitters, the p-type semiconductor region 7and the n-type semiconductor region 11 is preferably comparable to theones shown in FIG. 1a, FIG. 2a, FIG. 3a or FIG. 4a. Also in thisembodiment of the invention, the size and doping profile of the p-typesemiconductor regions 7 and the n-type semiconductor region 11 arepreferably equal or very similar to each other in order to provide thesame electron beam current values for the same voltages V1 and V2.Preferably, the region between the emitter tips 9 and the extractingelectrode 5 is at a good vacuum 27 in order to avoid deterioration ofthe performance of the emitter tips 9. Preferably, the vacuum 27 isbetter than 10⁻⁶ mbar and preferably better than 10⁻⁸ mbar.

[0155] The electron beam device 1 further comprises an extractingelectrode 5 which serves as an extracting electrode for the fieldemission cathodes 3. Therefore, in this embodiment, all emitter tips 9see the same voltage at the extracting electrode 5. Further in thisembodiment, the first voltage V1 between extracting electrode 5 and thep-type semiconductor region 7 is the same for all field emissioncathodes 3. It is provided by the first voltage source 21, which iselectrically connected to the p-type semiconductor regions 7 and theextracting electrode 5 through the conducting lines 25.

[0156] Preferably, the first voltage V1 is so high that the fieldemission cathodes 9 are operated in the saturation mode. In thesaturation mode the electron beam current 19 is almost independent ofchanges of the voltage between emitter tip 9 and extracting electrode 5.This increases the stability of the electron beam currents 19.

[0157] In the saturation mode the current control is performed by thesecond voltage V2 between p-type semiconductor region 7 and n-typesemiconductor region 11. For the same reason the current of the electronbeam depends only a little or not at all on the detailed structure ofthe emitter tips 9. This fact is a significant improvement overtraditional arrays of field emission devices. In the saturation mode theeffect of unavoidable manufacturing variations of the emitter tips donot significantly influence the electron emission rate behavior. Thismakes it possible, e.g., to operate large arrays of field emissioncathodes 3 with only one first voltage source 21 with a high degree ofelectron emission rate homogeneity.

[0158] The electron beam device 1 of FIG. 10a further comprises secondvoltage sources 23 for each field emission cathode 3 in order to provideindividual second voltages V2 across each pn-diode junction 13. Thisway, in the saturation mode, the currents of the electron beams 19 canbe individually controlled. This concept allows each field emissioncathode to be addressed individually to, e.g. switch the electron beams19 of each field emission cathode 3 either on or off or, increase ordecrease the electron beams 19 of each field emission cathode 3individually. Such electron beam devices may be useful for electron beampattern generators where the array of electron beams 19 is used tostructure a surface of a specimen with high throughput. It may also beuseful for flat panel displays where a structure of different brightnessis to be generated by the electron beams on a screen.

[0159] The conducting lines 25 and the second voltage sources 23preferably are integrated on the semiconductor substrate 37 usingmicromechanical techniques. Preferably the second voltage sources 23 areeach integrated right next to the corresponding field emission cathode3. This saves space and avoids long conducting lines. However if thespace between neighboring field emission cathodes 3 is too small, i.e.smaller than a few micrometers, there may not be enough space left tointegrate the second voltage sources 23 right next to each fieldemission cathode 3. In this case the second voltage sources V2 arepreferably integrated into the semiconductor substrate 37 outside thearray of field emission cathodes or even outside the semiconductorsubstrate 37. In this case, the conducting lines 25 for each have to beled from the field emission cathodes 3 outside the array of fieldemission cathodes 3 in order to provide the electrical connections tothe second voltage sources 23.

[0160] In FIG. 10b another embodiment of an electron beam deviceaccording to the invention is shown which is similar to the one shown inFIG. 10a. The main difference to the electron beam device shown in FIG.10a is the omission of the individual n-type semiconductor regions 11which instead have been merged with an n-type semiconductor substrate.As a consequence the n-type semiconductor regions 11 are electricallyconnected to each other and therefore are at the same electricalpotential with respect to the p-type semiconductor regions 7. Thisdesign simplifies the complexity of the array of field emission cathodesconsiderably since only one second voltage source 23 has to be providedinstead of one for each field emission cathode 3. For thousands or evenmillions of field emission cathodes 3 on a semiconductor substrate sucha simplification can be decisive for the success of an application.

[0161] On the other hand, with only one second voltage source 23 for allfield emission cathodes 3 there is no individual electron emissioncontrol possible any more. This may exclude the electron beam device 1from some applications. However electron beam devices such as electronmicroscopes, which need parallel electron beams 19 with constant andpossibly homogeneous electron beam currents, the simplification isuseful. The simplification is also important when a packaging density ofthe field emission cathodes 3 is needed which does not allow for anycircuitry between neighboring field emission cathodes 3.

[0162] In FIG. 10b an additional feature is shown which can be usefulfor some electron beam devices 1. In one of the field emission cathodes,i.e. the field emission cathode 3 a, the p-type semiconductor region 7has been increased in order to increase the minimum non-depleted p-typedistance D (see FIG. 1a). As mentioned before the minimum non-depletedp-type distance D determines the fraction of the injected electrons thatcan be emitted into free space. By increasing the minimum non-depletedp-type distance D the current of the electron beam 19 of that fieldemission cathode 3 is reduced to a electron beam current 19a. Therefore,it is possible to have individual electron beam current values with onlyone second voltage source 23 using layout techniques. However theelectron beam current values cannot be controlled individually duringoperation.

[0163] In FIG. 10c another embodiment of an electron beam deviceaccording to the invention is shown which is similar to the one shown inFIG. 10b. The main difference to the electron beam device shown in FIG.10b is the merging of the individual p-type semiconductor regions 7 toone p-type layer 7. With the p-type semiconductor regions being oneelectrically conducting p-type semiconductor layer 7, only one firstelectric contact 15 and only one second electric contact 17 are neededto bias all pn-diodes of the array of field emission cathodes. Thislayout further increases the potential for increased packaging of thefield emission cathodes since no conducting lines 25 are needed any morewithin the array of field emission cathodes. With this design it ispossible to have a spacing between neighboring field emission cathodesof less than a micrometer.

[0164] In addition, an array of field emission cathodes like in FIG. 10cneeds less manufacturing steps since the structuring of the p-typesemiconductor regions or n-type semiconductor regions can be omitted.This helps to reduce costs and improves manufacturing yield.

[0165] In FIG. 10d an electron beam device with an array of fieldemission cathodes 3 is shown which for the purpose of illustrationcombines many of the features mentioned in this description. Such adevice can be used, e.g., for electron microscopes with high throughput,where arrays of electron beams 19 with a single well-determined electronbeam current value have to be passed through focussing and directingcomponents 34 onto e.g. a specimen. For such applications individualelectron beam current control is not needed. Instead, high homogeneityof the electron beam currents and good current stability is appreciated.

[0166] Shown in FIG. 10d is an array of field emission cathodes 3 wherethe n-type semiconductor regions 11 have been merged with an n-typesemiconductor substrate, and where the p-type semiconductor regions 7have been merged to a p-type semiconductor layer 7. This design hasalready been described in FIG. 10c. It allows an array of electron beams19 to be generated with high current homogeneity across the array,however without individual current control. Such a design allows thearray pn-diodes of each field emission cathode 3 to be biased with onlyone second voltage source 23.

[0167] In addition, the extracting electrodes 5 have been integratedonto the substrate, preferably using microprocessing techniques. Usingmicroprocessing techniques, the extracting electrodes 5 have beenapplied on a structured insulating layer 40. Microprocessing techniquesallow the extracting electrodes 5 to be positioned very close to theemitter tips 9 with high precision. Using microprocessing techniques thedistances between the apex of emitter tips 9 to the extractingelectrodes 5 can be made as small as one micrometer or less. This allowsthe field emission cathodes 3 in the saturation mode to be operated at amoderate first voltage V1, e.g. less than 100 V. In addition, the highprecision of microprocessing techniques allows the field emissioncathodes 3 and extracting electrodes 5 to be fabricated with highgeometric homogeneity across the array of field emission cathodes.

[0168] Preferably the extracting electrodes 5 are electrically connectedto each other in a way that they are at the same electric potential.This can be achieved, e.g., by providing conducting lines betweenneighboring extracting electrodes 5. In another preferred embodiment,the extracting electrodes are made of a conducting layer with openings 6at the positions of the emitter tips 9. This way the first voltage V1between extracting electrodes 5 and p-type semiconductor regions 7 canbe provided by a single first voltage source 21.

[0169] In addition to the extracting electrodes 5 an anode 32 isprovided which preferably is at an electric potential more positive thanthe electric potential of the extracting electrodes 5. The anode 32serves to guide the array of electron beams 19 through the openings 6 ofthe extracting electrodes 5 towards, e.g., the anode 32. The electricpotential at the anode is provided by the third voltage source 30 whichdelivers a third voltage V3 between the extracting electrodes 5 and theanode 32.

[0170] In addition to the anode the focussing components 34 are shownwhich represent an optical system for the electron beams 19. Thefocussing components 34 usually comprise electric or magnetic componentsto direct or focus the electron beams 19. In this preferred embodimentof the present invention the array of electron beams 19 is focussed ontoan array of focus positions 35 which in this embodiment is on a planewith the anode 32. It is a major advantage of the present invention thatwith an array of field emission cathodes 3 like in FIG. 10d a highcurrent homogeneity is achieved. In addition, adjustments of theelectron beam currents are being performed by changes of the secondvoltage V2 which do not affect the performance of the optical systemrepresented by the focussing components 34.

[0171] From reading the present disclosure, other modifications andvariations will be apparent to persons skilled in the art. Suchmodifications and variations may involve equivalent features and otherfeatures which are already known in the art and which may be usedinstead of or in addition to features already disclosed herein. Althoughclaims have been formulated in this application to particularcombinations of features, it should be understood that the scope of thedisclosure of the present application includes any and every novelfeature or any novel combination of features disclosed herein eitherexplicitly or implicitly and any generalization thereof, whether or notit relates to the same invention as presently claimed in any claim andwhether or not it mitigates any or all of the same technical problems asdoes the present invention.

1. Electron beam device with a field emission cathode and an extractingelectrode, wherein the field emission cathode comprises: a p-typesemiconductor region connected with an emitter tip made of semiconductormaterial, wherein an electron current entering the emitter tip flowsthrough the p-type semiconductor region; an n-type semiconductor regionforming a pn-diode junction with the p-type semiconductor region; afirst electric contact on the p-type semiconductor region; and a secondelectric contact on the n-type semiconductor region.
 2. The electronbeam device according to claim 1, wherein the electron current enteringthe emitter tip flows through a non-depleted p-type portion.
 3. Theelectron beam device according to claim 1, wherein the maximum length ofthe minimum non-depleted p-type distance D during operation is shorterthan the diffusion length, L_(n), and preferably 10 times shorter thanthe diffusion length, L_(n), of the p-type semiconductor material. 4.The electron beam device according to claim 1, wherein the emitter tipis made of p-type material.
 5. The electron beam device according toclaim 1, wherein a positive first voltage (V1) between the extractingelectrode and the first electric contact is provided.
 6. The electronbeam device according to claim 1, wherein a forward biasing secondvoltage (V2) between the first electric contact and the second electriccontact is provided.
 7. The electron beam device according to claim 1,wherein the field emission cathode is integrated onto a semiconductorsubstrate.
 8. The electron beam device according to claim 7, wherein theextracting electrode is integrated onto the semiconductor substrate. 9.The electron beam device according to claim 1, wherein the extractingelectrode has an opening through which an emitted electrons beam canpass.
 10. The electron beam device according to claim 1, whereinfocusing components focus the electron beam.
 11. The electron beamdevice according to claim 1, wherein the emitter tip is coated withcoating material.
 12. An electron beam device comprising an array offield emission cathodes with an array extraction electrodes according toclaim
 1. 13. The electron beam device according to claim 12, wherein thearray of field emission cathodes is integrated onto a substrate.
 14. Theelectron beam device according to claim 12, wherein the extractingelectrodes are electrically connected with each other.
 15. The electronbeam device according to claim 12, wherein the n-type semiconductorregions are electrically connected with each other.
 16. The electronbeam device according to claim 12, wherein the p-type semiconductorregions are electrically connected with each other.
 17. The electronbeam device according to claim 12, wherein the p-type semiconductorregions are doped silicon material.
 18. A method to generate at leastone electron beam comprising: providing a field emission cathode and anextracting electrode, wherein the field emission cathode comprises: ap-type semiconductor region connected with an emitter tip made ofsemiconductor material, wherein an electron current entering the emittertip flows through the p-type semiconductor region; an n-typesemiconductor region forming a pn-diode junction with the p-typesemiconductor region; a first electric contact on the p-typesemiconductor region; and a second electric contact on the n-typesemiconductor region; applying a positive first voltage (V1) to theextracting electrode with respect to emitter tip; and applying a secondvoltage (V2) to the pn-diode junction.
 19. The method according to claim18, wherein the second voltage (V2) biases the pn-diode junction inforward direction.
 20. The method according to claim 18, wherein avacuum is generated between the extracting electrode and the emittertip;
 21. The method according to claim 18, wherein the electron currententering the emitter tip flows through a non-depleted p-type portion.22. The method according to claim 18, wherein the first voltage (V1) isat a level where the field emission cathode operates in the saturationmode.
 23. The method according to claim 18, wherein the minimumnon-depleted p-type distance D during operation is shorter than thediffusion length, L_(n), and preferably 10 times shorter than thediffusion length, L_(n), of the p-type semiconductor material.
 24. Themethod according to claim 18, wherein the emitter tip is made of p-typematerial.
 25. The method according to claim 18, wherein the fieldemission cathode is integrated onto a semiconductor substrate.
 26. Themethod according to claim 25, wherein the extracting electrode isintegrated onto the semiconductor substrate.
 27. The method according toclaim 18, wherein an array of field emission cathodes is integrated ontothe semiconductor substrate capable of generating an array of electronbeams.
 28. The method according to claim 27, wherein the second voltages(V2) are controlled individually.
 29. The method according to claim 18,wherein the p-type semiconductor region is doped silicon material. 30.The method according to claim 18, wherein the emitter tip is coated withcoating material.
 31. A field emission cathode comprising: a p-typesemiconductor region connected with an emitter tip made of semiconductormaterial, wherein an electron current entering the emitter tip flowsthrough the p-type semiconductor region; an n-type semiconductor regionforming a pn-diode junction with the p-type semiconductor region; afirst electric contact on the p-type semiconductor region; and a secondelectric contact on the n-type semiconductor region.
 32. A fieldemission cathodes comprising: a p-type semiconductor region connectedwith an emitter tip made of p-type semiconductor material, whereinessentially all electrons entering the emitter tip flow through thep-type semiconductor region; an n-type semiconductor region forming apn-diode junction with the p-type semiconductor region; a first electriccontact on the p-type semiconductor region; and a second electriccontact on the n-type semiconductor region.
 33. The field emissioncathode according to claim 31, wherein the electron current entering theemitter tip flows through a non-depleted p-type portion.
 34. The fieldemission cathode according to claim 31, wherein the minimum non-depletedp-type distance D during operation is shorter than the diffusion length,L_(n), and preferably 10 times shorter than the diffusion length, L_(n),of the p-type semiconductor material.
 35. Field emission cathodeaccording to claim 31, wherein the emitter tip is made of p-typesemiconductor material.
 36. Field emission cathode according to claim31, wherein the field emission cathode is integrated with asemiconductor substrate.
 37. Field emission cathode according to claim36, wherein an extracting electrode is integrated onto the semiconductorsubstrate.
 38. Field emission cathode according to claim 37, wherein theextracting electrode has an opening through which an emitted electronsbeam can pass.
 39. Field emission cathode according to claim 31, whereinthe emitter tip is coated with coating material.
 40. An array of fieldemission cathodes comprising field emission cathodes according to claim31.
 41. The array of field emission cathodes according to claim 40,wherein the array of field emission cathodes is integrated onto asubstrate.
 42. The array of field emission cathodes according to claim40, wherein the extracting electrodes are electrically connected witheach other.
 43. The array of field emission cathodes according to claim40, wherein the n-type semiconductor regions are electrically connectedwith each other.
 44. The array of field emission cathodes according toclaim 40, wherein the p-type semiconductor regions are electricallyconnected with each other.